Tito Ortiz -- Jenna Jameson's LYING ... I'm Not a Drug-Abusing Wife-Beater

UFC legend Tito Ortiz claims Jenna Jameson is lying through her teeth ... telling TMZ her allegations that he physically attacked her while abusing drugs are complete B.S..

Jenna went on a Twitter rampage yesterday ... claiming Tito beat her up and "almost killed" her ... and "faked" drug tests in order to fight in the UFC.

Jenna tweeted several pictures of prescription drug bottles and syringes -- insinuating they belonged to Tito.

During the Twitter session, Jenna also claimed Tito is unfairly trying to strip her of custody of their 4-year-old twin boys.

Now, Tito is finally responding to Jenna's allegations ... telling us, "The unfortunate allegations being made against me are simply not true."

He adds, "I would never do anything to jeopardize my family. Being a great father and protecting the well-being of my children remains my top priority."

"I thank my family, friends, and fans for their kind words of support at this time. For my children's sake, I appreciate you respecting our privacy."

As we reported, Jenna and Tito's twins are currently under the sole care of their father following a nuclear confrontation over custody last month.